I have already completed my project using BabylonJS. Now, I want to develop a mobile version that supports WebXR, especially AR. Which technology stack would be suitable for this?
I have seen many cross-platform framework solutions, but I am unsure about their support for WebXR. Additionally, I have found that many Android devices do not even support ARCore. Is there a way to make them compatible as well?
Unfortunately, WebXR isn’t going to support AR on iOS devices in most of the world. You might be able to use Babylon Native to create native XR apps with your existing code base. Otherwise, there are a number of platforms out there such as 8th Wall and Variant Launch that can offer a PWA like WebXR capability. These tend to be pricey (around $100) month, but they are the closest thing you can currently get to WebXR on iOS.
I believe you are confusing WebXR with Browser-based AR solutions. WebXR is a native layer iplemented by the browser. It can be mocked, but usually AR solutions like 8th wall (for example) don’t support WebXR or implement its API, but provide their own proprietary API.
WebXR is supported on most (new) Android devices. It is true that some older devices won’t have AR enabled, but I assume that AR solutions that are JS-based won’t run very well on those anyhow.
@ericwood73 is 100% correct in everything 
Apple doesn’t support WebXR on iOS.
I could be reading it wrong, but does it sound like Variant Launch (https://launch.variant3d.com/) works directly with the BabylonJS WebXR API? Meaning you can write code that would run in WebXR on Android and using Variant Launch that same code could work on iOS via app clips?
Yep, Variant is a different approach. It is an application, wrapping WebXR application, running natively on your phone without installation. Great idea!